The present invention relates to the field of combustion technology. It relates to a method for active suppression of hydrodynamic instabilities in a combustion system. It also relates to a combustion system for carrying out the method.
Thermoacoustic oscillations represent a danger to all types of combustion applications. They lead to high-amplitude pressure fluctuations, to constriction of the operational range, and can increase undesirable emissions. This affects, in particular, combustion systems with little acoustic damping, such as those used in gas turbines. Active control of the combustion oscillations may be required to guarantee high performance with regard to pulsations and emissions over a wide operating range.
Various techniques for controlling and suppressing combustion instabilities by means of an active control system have already been proposed, in which, using either an open or a closed controller, the supply of fuel and/or combustion air to the burner or to the burners is controlled or modulated in a defined manner. A prior, not previously published application from the applicant relates, for example, to active control of the instabilities in a premixing burner and is illustrated, for example, in FIG. 1 of EP-B1-0 321 809. In such a premixing burner, the fuel flows in the two outer fuel lines (8, 9 in FIG. 1 of EP-B1-0 321 809) are modulated asymmetrically in an open loop at frequencies between 0.3 Hz and 5 kHz, preferably between 5 Hz and 200 Hz. The modulation process is carried out with the aid of two fuel valves which are inserted in the fuel line.
A disadvantage with the use of mechanically moving, electrically driven fuel valves is that they have mechanically moving parts which are subject to increased wear at the modulation frequencies that are used, and whose functional reliability is subject to restrictions. Another disadvantage is the power required by the valves themselves, which makes additional circuit measures necessary.
The object of the invention is thus to specify a method for active control of combustion instabilities, which is simple and functionally reliable and presents only minor requirements in terms of hardware preconditions.
The essence of the invention is to use fluidics methods rather than unreliable mechanically operated valves for modulation of the fuel supply, that is to say to vary the fuel flows by hydrodynamic means without any moving parts, by using fluidic switches and control elements.
One preferred embodiment of the method according to the invention is distinguished in that, within the combustion system, the fuel is passed to two separate fuel lines for premixing, and in that, in order to modulate the supplied fuel, the fuel mass flow is alternately split in a different manner between the two fuel lines by fluidics means. Such alternate splitting is particularly suitable for premixing burners of the type mentioned above since this advantageously results in the axial symmetry of the combustion flame being disturbed and the axial symmetrical vortex structures and pressure fluctuations associated with axial symmetry being suppressed, or their creation being prevented. The alternate splitting can, for example, be achieved by supplying a first unmodulated partial mass flow of fuel equally via the two fuel lines, while a second partial mass flow is additionally supplied alternately via one of the two fuel lines. This process does not utilize the full modulation depth in the fuel supply.
However, it is also conceivable, according to a preferred development of the embodiment, for the (entire) fuel mass flow to be passed alternately via one of the two fuel lines (full modulation depth).
The modulation process is preferably carried out using a periodic time function, at a predetermined frequency and with a predetermined amplitude. The frequencies are in this case governed by the geometry and method of operation of the combustion system, and are normally in a range which has already been mentioned further above in conjunction with the prior art.
The destruction of the symmetries in the flame or combustion chamber which promote oscillations can in this case be achieved on the one hand by the fuel being passed via the two fuel lines to a single premixing device and being injected at different points there.
However, it is also conceivable for the fuel to be passed via the two fuel lines to different premixing devices (for example premixing burners) within the same combustion system and to be injected there, which leads to symmetry suppression within the entire system comprising a plurality of premixing devices.
In the combustion system according to the invention, which comprises a premixing device for mixing the fuel with the combustion air, at least one fuel line for supplying the fuel to the premixing device, and means for modulation of the mass flow of the supplied fuel, is distinguished in that the modulation means comprise a fluidics element.
Another preferred embodiment of the combustion system according to the invention is distinguished in that the fuel is supplied via two fuel lines and in that the fluidics element is designed and is connected to the two fuel lines such that, when modulation occurs, at least a portion of the supplied fuel mass flow is switched alternately to one of the two fuel lines. In particular, the two fuel lines lead to the same premixing device, and the premixing device is designed such that the fuel from each of the fuel lines is injected at a different point in the premixing device.
The fluidics element which is used preferably comprises a fuel inlet and two fuel outlets which branch in a Y-shape from the fuel inlet and are connected to the fuel lines, and two mutually opposite control channels, which run transversely with respect to the fuel inlet, that open into the fuel inlet in the region of the branch of the fuel outlets. By applying increased pressure or reduced pressure, the element allows the fuel mass flow entering the fuel inlet to be diverted from one fuel outlet to the other.
The desired modulation is achieved in a particularly simple manner with the aid of this fluidics element if the two control channels are connected to one another in a closed circuit by means of a connecting tube of predetermined length running outside the fluidics element.